Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sun Jan 24 2021 Valid 12Z Wed Jan 27 2021 - 12Z Sun Jan 31 2021 ...Heavy precipitation likely for California mid-late week with another possible event focusing along the central West Coast next weekend... ...Rapidly deepening low pressure could bring strong winds and a quick round of wintry weather across the Mid-Atlantic next Thursday... ...Weather Pattern Overview and Guidance/Predictability Assessment... An active synoptic pattern will likely continue into the medium-range period. Guidance this morning showed relatively good agreement nonetheless. Since yesterday, models have come into reasonably good agreement on a rapidly intensifying storm early next Thursday near the Mid-Atlantic coast before moving relatively quickly out to sea. The GFS tends to intensify the storm closer to the North Carolina coast while the ECMWF is slightly farther offshore. The CMC has adopted a slower progression with this system and it is on the slow end of the guidance envelop. The consensus of the GFS/GEFS and the ECMWF/EC mean appears to offer a good starting point that maintains good continuity with previous forecasts. On the West Coast, models have settled into a very good agreement on the timing of the next heavy precipitation event forecast to be well under way across northern to central California by Wednesday. This system should then gradually weaken as it slowly slide southeastward into southern California by Friday with lingering precipitation over much of California. By next weekend, another developing trough farther offshore should direct moisture more toward the central West Coast. Meanwhile, upper-level shortwaves moving across the Rockies are forecast to eject into the Plains during next weekend. Model consensus is now indicating a more consolidated low pressure system to track across the central Plains toward the Great Lakes by the weekend as guidance this morning indicated a stronger high pressure system over central Canada late next week than previous forecasts. The WPC medium-range forecast package was based on the consensus of the 06Z GFS/GEFS, the 00Z ECMWF/EC mean. The CMC was not included since it is not as compatible with the GEFS/EC mean consensus than it was yesterday. More of the ensemble means were used on Days 6 and 7 to handle the increasing model spread. ...Weather/Threats Highlights... Expect the upper trough sharpening just off the West Coast along with the associated low pressure/frontal system to support areas of heavy rain and mountain snow over California in the mid-late week time frame. There will likely be a gradual southward progression from the central Coastal Ranges and Sierra Nevada into the southern Coastal and Transverse Ranges. Guidance continues to suggest the potential for some 5-10 inch liquid totals in this event. Less extreme precipitation totals will spread through other parts of the West and into the Rockies. Acceleration of the upper trough should produce a surface system and expanding precipitation area from the Plains eastward next weekend. A warming trend ahead of this system should limit most of any wintry weather to the Great Lakes/central Appalachians/Northeast. Also next weekend upstream Pacific flow should begin directing another round of moisture into the West Coast. Currently the best signal for highest totals with this next event exists over northern California and southwestern Oregon with somewhat lesser amounts farther north over the Pacific Northwest. Although guidance has not yet fully converged, agreement is improving for the evolution of initial Gulf Coast/southern U.S. low pressure that should deepen rapidly as it tracks into and over the western Atlantic. This storm should be just off the Mid-Atlantic coast by early Thursday and then continue farther offshore with time. Currently expect the best potential for some snow to extend from the Middle Mississippi Valley through the Mid-Atlantic with highest totals over the central Appalachians/Mid-Atlantic. Finer details that can take into the short range to be resolved will ultimately determine accumulations at any particular location. Progressive nature of this storm may be a limiting factor for snowfall totals. Meanwhile this storm may bring an area of moderate to heavy rainfall to the Southeast. Consensus still shows a period of strong winds along and offshore the East Coast in the gradient between the deepening storm and trailing high pressure building toward the eastern states. Ahead of the Gulf Coast wave that will deepen rapidly as it moves into the western Atlantic, lows over the Southeast will be quite warm to start the period early Wednesday--as much as 15-25F above normal. Cool high pressure building into the East behind this developing storm will bring readings down to moderately below normal levels for a couple days late this week. Meanwhile around midweek the Southwest and vicinity may see highs as much as 10-15F below normal with parts of the central Plains 10-20F below normal. The southwestern states will moderate slightly after Wednesday but likely remain below normal. On the other hand the upper ridge building into the central and eventually eastern U.S. will promote a warming trend to areas east of the Rockies. The northern Plains will see plus 10F or greater anomalies first, followed by an expanding area to the south and east. By the weekend an increasing proportion of the eastern two-thirds of the U.S. should see plus 10-25F anomalies for lows and some areas of plus 10-15F anomalies for highs. Kong/Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml